1. Field of the Invention
The present invention relates to an aberration compensating optical element having a diffractive structure, an optical system comprising this aberration compensating optical element, an optical pickup device, a recorder and a reproducer. Further, the present invention relates to an aberration compensating optical element which is disposed on an optical path between a light source and a high NA objective lens having at least one plastic lens and which can minimize a change in a spherical aberration of the objective lens, which is caused by a temperature change; an optical system comprising this aberration compensating optical element and a high NA objective lens having at least one plastic lens, wherein the optical system is used for an optical pickup device for carrying out at least one of the record of information on an optical information recording medium and the reproduction of information from an optical information recording medium; an optical pickup device comprising this optical system, for carrying out at least one of the record of information on an optical information recording medium and the reproduction of information from an optical information recording medium; a recorder comprising this optical pickup device, for carrying out the record of at least one of a sound and an image on an optical information recording medium; and a reproducer for carrying out the reproduction of at least one of a sound and an image from an optical information recording medium.
2. Description of Related Art
In recent years, a new high density storage optical pickup system using a blue-violet semiconductor laser light source having an oscillation wavelength λ of about 400 nm and an objective lens having an enhanced numerical aperture (NA) of about 0.85, has been researched and developed. On an optical disk having a diameter of 12 cm, which is the same as that of a DVD (NA=0.6, λ=650 nm, storage capacity 4.7 GB), for example, an optical pickup system (NA=0.85, λ=400 nm) can record 25 GB of information.
However, when such a high NA objective lens and a short wavelength light source having the oscillation length of about 400 nm are used, a problem that an axial chromatic aberration occurs at an objective lens, is caused. In general, a laser light emitted from a semiconductor laser has a single wavelength (single mode), and it is thought that the axial chromatic aberration does not occur. In practice, there is some possibility that the mode hopping in which a center wavelength of a laser light is instantly hopped about several nanometers by changing the temperature, the output of the light or the like, occurs. Because the mode hopping is a phenomenon that a wavelength is changed so instantly that the focusing of an objective lens cannot follow the wavelength change, if a chromatic aberration occurring due to the objective lens is not compensated, a defocus component caused by the mode hopping is added. As a result, a wavefront aberration increases. In case of using a high NA objective lens or a short wavelength light source, the wavefront aberration caused at the mode hopping specially increases for the following reason. When due to a wavelength change Δλ, the spherical aberration is not changed at the objective lens and the back focus fb is changed by Δfb, if the objective lens is focused in an optical axis direction against a change of the back focus, the root mean square value Wrms of the wavefront aberration is 0. When the objective lens is not focused, the value Wrms is expressed by the following Formula (1).Wrms=0.145·{(NA)2/λ}/|Δfb|  (1)
For example, when an optical pickup system for DVD (NA=0.6, λ=650 nm) is compared with one for an optical disk (NA=0.85, λ=400 nm), in case of the same Δfb, the wavefront aberration occurring in the latter optical pickup system increases by 3.26 times. That is, if the permissible value of the wavefront aberration is the same in both systems, the permissible value of |Δfb| decreases by 1/3.26 in the latter system. Therefore, it is required that the axial chromatic aberration on a wave surface of a light which transmits through the objective lens and is condensed on a storage surface of an optical disk, should be small.
In such a high density optical pickup system, in order to save the cost thereof and to lighten it, it is desired that an objective lens is a plastic lens like a conventional CD system and a DVD system. However, in a high NA plastic objective lens, when the temperature changes, the change in the spherical aberration, which is caused by the refractive index change of plastic, is large because it increases in proportion to the fourth power of NA. Therefore, in practical use, the above change becomes a significant problem.
As a cemented doublet type of objective lens for optical disk, which is formed so as to compensate the chromatic aberration, ones disclosed in Japanese Patent Publications (laid-open) No. Tokukai-Sho 61-3110 and No. Tokukai-Sho 62-286009, are known. The above lens which is formed by combining a lens made of low dispersion material having a positive refractive power and a lens made of high dispersion material having a negative refractive power, is unsuitable for an objective lens used for a optical disk, which is required to be light. Because there is a limit of the dispersion of the material, the above lens itself becomes heavy in order to obtain a high NA and to increase the refractive power of each lens.
In Japanese Patent Publication (laid-open) No. Tokukai-Hei 11-174318, an objective lens having a doublet lens structure, wherein the numerical aperture on an optical disk side is 0.85, and a hologram is provided on an optical surface in order to compensate the axial chromatic aberration, is disclosed. However, when the hologram comprises a ring-shaped zone structure having a plurality of fine steps which are formed into concentric circles, in a high objective lens which has a tendency to decrease a curvature of the optical surface, a shadow of the ring-shaped zone structure largely influences the transmitted light and then the transmittance of the light decreases. Therefore, the above objective lens is unsuitable for a high density storage optical pickup system as an information writing system which is required to have a high light utilization efficiency.
As an aberration compensating optical element for compensating the axial chromatic aberration of the objective lens, one disclosed in Japanese Patent Publication (laid-open) No. Tokukai-Hei 6-82725, is known. When the aberration compensating optical element in which a plurality of steps are formed as ring-shaped zones having the form of concentric circles around an optical axis on a plane surface perpendicular to the optical axis, is disposed in parallel light flux, a reflected light in a diffractive structure returns in the same direction as an incident light. As a result, a ghost signal is generated in a detection system of the optical pickup device. Therefore, The above lens is unsuitable for an aberration compensating optical element used in an optical pickup system.